Question

#ifdef __cplusplus
// C++ code
#else
// C code
#endif

The structure is this.
My question is, how to actually trigger the #ifdef on?

I mean, in program? What code I write can turn #ifdef on?

For example, in this case.
is that

#define __cplusplus

will turn it on?

Answer 1

“#define __cplusplus”

will let it on?

Yes, it will “let it on”.

__cplusplus should be automatically defined by C++ compiler. C++ uses different name mangling and the macro often used to make C headers compatible with C++:

#ifdef __cplusplus
extern "C" {
#endif

...

#ifdef __cplusplus
}
#endif